1. Field of the Invention
The present invention relates to an electronic control system for controlling fuel injection in internal combustion engines of the fuel injection type and, more particularly, an electronic control system of the feedback type employing a means for sensing the air/fuel ratio.
2. Description of the Prior Art
Electronic control systems for controlling fuel injection in internal combustion engines of the fuel injection type generally include means for computing the timing of the fuel injection so as to match the requirements of the engine in accordance with various input signals such as a trigger signal delivered from the distributor indicating the rotational speed of the engine, a signal taken from an intake vacuum sensor or an intake air flow sensor which represents engine load and other signals taken from various sensors switches and indicating other operational conditions of the engine and means for producing a pulse signal having a pulse length corresponding to the computed time of fuel injection, this pulse signal being supplied to a power amplifier which produces power output pulses for actuating fuel injection nozzles.
Such an electronic control system is generally an open loop system in accordance with the classification in the art of control systems. In recent years, however, in accordance with the requirements for exhaust gas purification, fuel economy and drivability of automobiles, efforts have been made to provide an electronic control system of the closed or feedback type by incorporating an air/fuel ratio sensor which detects the air/fuel ratio of the intake fuel-air mixture from the quantity of combustible components and residual oxygen contained in the exhaust gases of the engine. In such systems an on-off signal delivered from the sensor is supplied as an input to an integration circuit and the output from the integration circuit modifies the pulse length of the aforementioned pulse signal. The air/fuel ratio sensor generally produces two kinds of discretely different output values in opposite regions of the stoichiometric air/fuel ratio so that it provides on or off signal on the rich side and off or on signal on the lean side. Therefore, the output of the air/fuel ratio sensor is generally supplied to an integration circuit before it is fed into a control circuit. When an electronic feedback control system of this type is employed, if the air/fuel ratio of the fuel-air mixture supplied to the engine has shifted to the lean or rich side of a predetermined air/fuel ratio in accordance with a variation of the operational condition of the engine, the change is detected by the air/fuel ratio sensor which then produces an on-off signal. The on-off signal effects modification of the pulse length of the pulse signal, whereby the quantity of fuel required for correcting the air/fuel ratio to the predetermined value is increased or decreased at a predetermined rate, thus recovering the predetermined air/fuel ratio. The rate at which the fuel is increased or decreased is determined by the integration constant of the integration circuit to which the on-off signal from the air/fuel ratio sensor is supplied. In this connection, since there is a delay in the response of the exhaust gases in the variation of air/fuel ratio when compared with that in the intake fuel-air mixture, the delay corresponding to the sum of the delay time of gases in the combustion chamber and the time required for exhaust gases to travel through the exhaust system from an exhaust port to the air/fuel ratio sensor, it is noted that the rate at which the fuel is increased or decreased should preferably be increased or decreased in accordance with the increase or decrease of intake air flow as shown in FIG. 1. However, since in the conventional system the integration time constant in the integration circuit is fixed at a constant value, the control of the controlling air/fuel ratio when the engine is in a transient operation is different from that in a steady running condition thereby causing some inconsistency in adjusting the control system for optimum operation, resulting in a relatively rough control in actual operation.